1. Field of the Invention
The present invention relates to a semiconductor device including an oxide semiconductor. Note that in this specification, a semiconductor device refers to a semiconductor element itself or a device including a semiconductor element. As an example of such a semiconductor element, for example, a transistor (a thin film transistor and the like) can be given. In addition, a semiconductor device also refers to a display device such as a liquid crystal display device.
2. Description of the Related Art
A thin film transistor formed over a flat plate such as a glass substrate is manufactured using amorphous silicon or polycrystalline silicon, as typically seen in a liquid crystal display device. A thin film transistor manufactured using amorphous silicon has low field effect mobility but can be formed over a larger glass substrate. In contrast, a thin film transistor manufactured using crystalline silicon has high field effect mobility, but, due to a crystallization step such as laser annealing, such a transistor is not always suitable for being formed over a larger glass substrate.
In view of the foregoing, attention has been drawn to a technique by which a thin film transistor is manufactured using an oxide semiconductor, and such a transistor is applied to an electronic device or an optical device. For example, Patent Document 1 and Patent Document 2 disclose a technique by which a thin film transistor is manufactured using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as an oxide semiconductor film and such a transistor is used as a switching element or the like of an image display device.
Semiconductor characteristics of such an oxide semiconductor are significantly influenced by an oxygen concentration in the oxide semiconductor. Therefore, in order to suppress variation in the characteristics of an oxide semiconductor, it is important to suppress oxidation-reduction reaction of the oxide semiconductor and keep the oxygen concentration. Thus, mixing of moisture into an oxide semiconductor has been prevented by providing an oxide film or a nitride film mainly including silicon over the oxide semiconductor.
However, by using a silicon oxide film or a silicon nitride film, it has been difficult to satisfactorily prevent moisture from being mixed into the oxide semiconductor. In addition, if the thickness of a silicon oxide film or a silicon nitride film is increased in order to prevent penetration of moisture, stress is increased and therefore a crack is easily caused.
Moreover, there is a problem in that semiconductor characteristics of an oxide semiconductor are changed when an impurity such as alkali metal (Li, Cs, Na, or the like), alkaline earth metal (Ca, Mg, or the like), or another metal element as well as moisture are diffused into the oxide semiconductor (see, for example, Patent Document 3).